1. Field of the Invention
The present invention relates to a motor control device provided with a voltmeter which detects the power supply voltage applied to a three-phase inverter (or bridge circuit), for drivingly controlling a brushless DC motor (or DC motor) by the use of the three-phase inverter (or bridge circuit). In particular, it relates to means for inferring the value of an electric current supplied to the three-phase inverter (or bridge circuit). The present invention further relates to an electric power steering device incorporating the aforementioned motor control device.
2. Discussion of the Related Art
Heretofore, as shown in FIG. 12, there has been used a motor control device 1000 for drivingly controlling a brushless DC motor (or a DC motor) 107 by the use of a three-phase inverter (or a bridge circuit) 105. In the motor control device 1000, a battery 101, the three-phase inverter (or the bridge circuit) 105 and a power supply current sensor 104 which is provided on a connection line for connection therebetween have generally been used as means for detecting the overcurrent supplied to the three-phase inverter (or the bridge circuit) 105.
However, the prior art as described above necessitates the provision of the power supply current sensor 104 and is unavoidable to be increased in the cost therefor. Further, the omission of the power supply current sensor 104 results in failure to detect the amount of the electric current supplied to the three-phase inverter (or the bridge circuit) 105. This makes it unable to detect the state of overcurrent, whereby an appropriate countermeasure becomes difficult to take upon occurrence on an abnormality.
Japanese unexamined, published patent application No. 2004-32848 (patent document 1) has been proposed to solve the foregoing problems. In the patent document 1, since an expression (4) (described paragraph 0008) takes the assumptions of Id*=0 and Vd*=0, the effective value of the electric power P becomes smaller that an actual value, whereby it is unable to detect the current value accurately. In addition, although a motor as the object to be controlled is limited to brushless DC motors in the patent document 1, DC motors have been in wide use practically and thus, it has been required to derive a calculation method for the DC motors.